The invention relates to a voltage converter comprising a half bridge of controlled semiconductor switches, a transformer and a rectifier circuit, a primary winding of the transformer in series with at least one capacitor being connected to the half bridge.
Voltage converters of the generic type are known for various purposes and in several versions. By connecting a rectifier in series, the voltage converters may also be used as a power supply unit for electronic devices. They are not only used for generating one or more DC voltages but also for the DC isolation of the mains supply and the powered circuits.
Such circuits are described in, for example, U.S. Pat. No. 5,808,879 and U.S. Pat. No. 5,402,329 and have characteristic features such as switching at zero-crossing, control by pulse-width modulation, fixed operating frequency, optimum resetting of the transformer core, utilization of the stray inductance of the transformer, reduced power load of the semiconductor circuit and low electromagnetic perturbing radiation. These circuits work with a largely constant voltage across the capacitor within a respective half wave, so that the capacitor is only used as a coupling capacitor so as to avoid a direct current showing up in the primary winding of the transformer. This is particularly described in U.S. Pat. No. 5,808,879, column 6, line 42. This document describes a linear current waveform (see U.S. Pat. No. 5,808,879, FIG. 5 and U.S. Pat. No. 5,402,329, FIGS. 6b to 6f).
It is an object of the present invention to provide a voltage converter that has increased efficiency and reduced cost compared with the known voltage converters.
According to the invention this object is achieved in that the at least one capacitor is designed such that the resonant frequency of the series combination comprising the at least one capacitor and an inductance that is active when the rectifier circuit is in operation is larger than half the pulse frequency with which the semiconductor switches are controlled. The active inductance may then be formed by the stray inductances of the transformer. If these inductances cannot meet the specific requirements, according to a further embodiment it could also be provided that a coil is connected in series with the transformer and the at least one capacitor. This coil may be situated on the primary side or secondary side.
With the invention the capacitor causes not only DC isolation to occur, but also influences the current path through the primary winding of the transformer, so that the peak currents are reduced and more cost-effective components can be used with the same output power or, while maintaining the components, a higher output power can be realized. The higher output power is achieved by the steeper rise of current at the beginning of the half cycle.
These properties permit the use of a transformer that has lower specific requirements, for example, a higher stray inductance.
An advantageous embodiment of the voltage converter according to the invention consists of the fact that a series combination of the primary winding and the at least one capacitor is connected between the tapping point and one of the end points of the half bridge. The capacitor may then be connected to the tapping pointxe2x80x94that is, the connection point between the semiconductor switchesxe2x80x94and the primary winding to the end point and the other way round.
Another advantageous embodiment consists of the fact that two capacitors form a further bridge arm and that the primary winding is connected between the tapping point of the half bridge and that of the further bridge arm.
The invention further relates to a voltage converter comprising a half bridge of controlled semiconductor switches, a longitudinal coil and a transverse coil being connected in series with at least one capacitor to the half bridge and a rectifier circuit being connected in parallel to the transverse coil. Such a voltage converter is described, for example, in Poon N. K. and Pong M. H.: xe2x80x9cA Novel ZVS Direct Coupling Converter (DCC)xe2x80x9d in IEEE PESC, 1996, pp. 94 to 99.
With a voltage converter of this type the object according to the invention is achieved in that the at least one capacitor is dimensioned such that the resonant frequency of the series combination of the at least one capacitor and the longitudinal coil is larger than half the pulse frequency with which the semiconductor switches are controlled.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment(s) described hereinafter.